Activated carbon materials are commonly used to adsorb hydrocarbons and other impurities from gas streams (frequently air) and liquids.
For these applications, the carbon is generally used in the form of granules. While activated carbon in the form of granules can perform the desired adsorption for many applications, there are some applications in which the granules have drawbacks. In some cases back pressure is a problem with the granules since the flow must follow a tortuous path. Some applications can result in considerable wear of the granules by attrition, causing loss of material or bed packing due to the fines resulting in the blocking of the flow.
Another approach is to use an extruded activated carbon in the form of a cellular structure such as a honeycomb. These structures can readily be shaped by extruding fine powders of activated carbon with suitable binders. The honeycomb shape allows for ease of flow of the gases therethrough with little back pressure. Also, the geometry can be such as to allow easy access of the gases to all of the carbon for adsorption of the species to be removed. In the use of granules, the adsorbing species must diffuse into the center of the granule. This diffusion distance can be great compared to the thickness of the web of a honeycomb. Also, since a honeycomb is a solid piece, there should be little or no wear or attrition of the carbon.
Among the uses for such activated carbon honeycombs are the adsorption of hydrocarbon vapors in automotive applications. There are two automotive applications: (1) the evaporative emissions of vapors from the fuel system and the engine intake areas, and (2) cold start application.
In the case of evaporative emissions, the activated carbon adsorbs vapors given off from the fuel system while the vehicle is not operating, such as from the expansion and contraction of gases in the fuel tank with temperature swings. During refueling, the air which is displaced from the tank carries along considerable fuel as vapors which must be captured to meet future air pollution standards. The adsorbed species is then desorbed while the engine is operating and recycled back into the engine intake to be burned. Most vehicles today have an activated carbon canister filled with the granules to take care of some sources of vapors. However, this is not adequate to meet future requirements.
In the cold start application the activated carbon adsorbs hydrocarbons emitted during the initial 90 to 120 seconds after start-up of the engine. During this start-up period, the catalytic converter is not up to temperature for converting the hydrocarbons being emitted from the engine. Once the catalytic converter is up to temperature, the activated carbon can be removed from the exhaust system in a by-pass mode. The adsorbed hydrocarbons are desorbed from the activated carbon and are fed into the engine or into the exhaust ahead of the catalytic converter where they are converted to innocuous entities. The activated carbon is thus ready to adsorb hydrocarbons during the next cold start cycle.
In order to form an activated carbon honeycomb by extrusion, the carbon must be in the form of a fine powder. This can then be mixed with a liquid such as water and suitable plasticizers and binders. This plasticized mixture is then extruded through a die into the honeycomb shape, and dried.
Organic binders such as methylcellulose provide plasticity to the mixture. Such mixtures are relatively soft and difficult to handle in the wet as extruded state before drying. Moreover, the bodies formed from such mixtures are relatively low in strength especially at elevated temperatures such as 250.degree. C. which are encountered in applications such as auto exhaust purification. This is a result of the degradation of the organic binders.
It is highly desirable to improve the strength of the extruded honeycomb both in the extruded state for further processing and handling and also after drying to improve performance.
Clays and resins have been used as binders in carbon mixtures to impart strength to the carbon body formed therefrom.
However, high levels of some binders result in decreased surface area in the body when used in the as-formed state. As a result, the adsorption efficiency of the activated carbon decreases. This is an important consideration in hydrocarbon adsorption applications.
U.S. Pat. Nos. 4,259,299, 4,518,704, and Japanese patent application publication no. 57-122924 (1982) relate to activated carbon bodies in which clay binders are used.
U.S. Pat. Nos. 4,259,299 relates to using bentonite in activated carbon-zeolite mixtures. The material is heated to 350.degree. C. to develop strength. Although some strength is developed, the material breaks down if in contact with water. This is because of the bentonite which absorbs water and expands causing complete breakdown of the structure. Japanese patent publication No. 49-115,110 (1974) relates to a carbon material honeycomb body made by mixing carbon material or carbonizable material, thermosetting resin, binder and suitable solvent to form a plastic mixture, extruding and firing. The binder can be a thermoplastic or thermosetting resin such as phenol, Formalin, etc., CMC, dextrin, etc. For increasing strength bentonite, kaolin, etc. clay materials can be mixed in.
There remains a need to produce activated carbon bodies of improved strength to allow them to function effectively in high temperature applications such as in auto exhaust purification applications without sacrificing surface area and hence adsorption efficiency. Also there is a need to produce activated carbon bodies which maintain their structural integrity when in contact with water.